Superconductivity has been discovered in the materials that form when alkali metals react with a solid hydrocarbon. This is the first new class of organic, high-temperature superconductor in a decade.
New high-temperature superconductors continue to arrive on the scene — the most recent noteworthy additions being the iron arsenides — but there have been no new organic superconductors the past decade. Now the discovery of superconductivity at temperatures up to 18 K is reported in crystals of a simple hydrocarbon molecule doped with potassium or rubidium. The basis for the new compound is picene (C22H14), a molecule consisting of five benzene rings sharing edges with one another, which crystallizes into an ordered molecular solid. Intercalation of the alkali metals into the crystal lattice induces metallic behaviour and superconductivity in what is normally a semiconducting material. The Tc of 18 K in potassium-doped picene is high for an organic superconductor — only alkali-metal-doped C60 achieves higher. And as picene is one of a large family of molecules based on fused benzene rings, other superconducting hydrocarbons may be awaiting discovery.
Efforts to identify and develop new superconducting materials continue apace, motivated by both fundamental science and the prospects for application. For example, several new superconducting material systems have been developed in the recent past, including calcium-intercalated graphite compounds1, boron-doped diamond2 and—most prominently—iron arsenides such as LaO1–xFxFeAs (ref. 3). In the case of organic superconductors, however, no new material system with a high superconducting transition temperature (T c) has been discovered in the past decade. Here we report that intercalating an alkali metal into picene, a wide-bandgap semiconducting solid hydrocarbon, produces metallic behaviour and superconductivity. Solid potassium-intercalated picene (Kxpicene) shows T c values of 7 K and 18 K, depending on the metal content. The drop of magnetization in Kxpicene solids at the transition temperature is sharp (< 2 K), similar to the behaviour of Ca-intercalated graphite. The T c of 18 K is comparable to that of K-intercalated C60 (ref. 4). This discovery of superconductivity in Kxpicene shows that organic hydrocarbons are promising candidates for improved T c values
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